Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - LightPipe (Antiresonant Hollow Optical Fibres for a Quantum Leap in Data and Optical Power Transmission)

Teaser

Fibre optics has revolutionised telecommunications, enabled the widespread diffusion of the internet and profoundly impacted industrial manufacturing, metrology, medical endoscopy and structural sensing, to name but a few. In many applications however, fibres are now being...

Summary

Fibre optics has revolutionised telecommunications, enabled the widespread diffusion of the internet and profoundly impacted industrial manufacturing, metrology, medical endoscopy and structural sensing, to name but a few. In many applications however, fibres are now being operated very close to fundamental physical limits of the glass that forms their core, and this is already providing hard limits, for example, to the maximum data capacity or optical intensity that can be transmitted through them. A transformative new technological step is required to help increasing the information capacity and power delivery capability of optical fibres to keep up with the 1.5dB/year growth in global data traffic and with the 2dB/year raise in laser output power.
Air guiding hollow core fibres can provide a natural solution, but the state of the art technology suffers from conceptual physical limitations that bound their minimum loss, maximum information capacity, and transmitted optical power and energy. This proposal addresses these global challenges by developing the ‘ultimate’ hollow core optical fibre technology based on nested antiresonant nodeless fibres.
Based on a recent discovery of the PI yet to find experimental demonstration, these fibres exploit antiresonances and multiple coherent reflections from the glass membranes to achieve, unlike any other known air-guiding optical waveguide, simultaneous minimisation of surface scattering and leakage loss. By targeting a 10 times increase in data capacity and power handling and a 5 times reduction in transmission loss as compared to state-of-the-art technology, all in an ultra-low nonlinearity fibre with excellent modal purity and spectral transparency, the outcomes of this project have the potential to revolutionise telecommunications 45 years after the development of ultra-low loss glass optical fibres and to produce a step-change in many industrial and scientific high power laser delivery applications. Besides impacting data communications and laser delivery for manufacturing, the outcome of this project have the potential to enable novel applications such as laser assisted drilling in oil wells, ultraprecise long distance transfer of timing signals and optical frequencies, laser spark ignition in highly efficient combustion engines and laser driven particle acceleration.

Work performed

When the proposal was written 3 years ago, the technology of Nested Antiresonant Nodeless Fibres (NANFs) did not exist. The whole proposal was based on a vast amount of carefully executed theoretical and numerical work; this predicated that if we found a way to develop hollow core optical fibres with a given structure, their performance would be rather remarkable and exploitable in many fields. Many challenges lied ahead regarding the fabrication of such structures.
One of the main outcomes of the first two and a half years of Lightpipe is that after a considerable amount of work and attempts, my team has now managed to develop a reproducible fabrication process to produce good quality NANFs. A few months ago we succeeded in developing a practical fabrication process and we are now in the position to produce several hundred metres of NANF from one preform (to be scaled to a few km over the course of 2019), and to shift the operational wavelength of the fibres arbitrarily to suit the demand of different applications.

The main published outcome of this work is the demonstration of a NANF with the lowest loss ever reported in a hollow core fibre (HCF). This was disseminated at the postdeadline session of the European Conference on Optical Communications in 2018 and broke the previous 14 year old world record obtained with a different technology (Photonic Bandgap Fibre (PBGF)). The fibre we disclosed had a minimum attenuation of 1.3 dB/km (previous record: 1.7dB/km). Besides, its bandwidth was well in excess of 100nm (vs 20nm), the surface mode that plague PBGF transmission were not present, and the fibre had an effectively single mode behaviour (vs a few-moded behaviour). As a result of such an excellent modal purity, which is an inherent feature of the NANF concept, we also demonstrated some preliminary data transmission (10Gb/s through 500m of fibre, with no penalty observed). For comparison, the best data-transmitting HCF ever reported in the 1550nm region was a PBGF with a loss of 3.5 dB/km. This is therefore a pretty significant step forward towards the demonstration of practical HCFs for data transmission and already makes this technology appealing for some applications like intra data-centre interconnections and 5G communications between antennas and optical access network.

For over two decades the fibre optics community had been of the opinion that use of PBGF technology was the way to go to achieve low loss in a hollow core fibre. Our recent result has proved that this is not the case. Besides, we have also reported an excellent agreement with modelling, resulting from the custom development and validation of several codes to model each individual loss contribution in the fibre (scattering, leakage, microbend, …). Importantly for Lightpipe, the same code that predicts a loss of the fabricated fibre in excellent agreement with the measurement, also predicts that by modifying certain structural parameters, a considerably lower loss could be achieved. Therefore, the ultimate aim of the project to produce fibres with a loss lower than that of standard silica fibres seems still achievable.

Final results

So far the Project has achieved and published the lowest loss ever reported in a data transmitting hollow core fibre, 1.3 dB/km. Although an impressive result, the attenuation is still a factor of ~8 higher than conventional single mode fibres at the wavelength of use for long distance optical communications (1550nm). The same fibre however had a loss of 1.8 dB/km at the shorter wavelengths typically used for short distance, intra-datacentre applications (850nm). At these wavelengths, the loss of conventional fibres is about 2.2 dB/km. This is therefore the first demonstration that a hollow core fibre can outperform standard optical fibre technology in well-established applications.
As the fabrication technology matures and fibres are beginning to be passed on to other scientists for exploratory works in other application domains, remarkable and occasionally unexpected results start to emerge. These have not yet been made public, but will become more and more the focus of the project in its second half.
From now till the end of the project we expect breakthroughs in:
• further loss reduction: demonstration of a hollow core waveguide with lower loss than any other optical fibre before
• power delivery: demonstration of multi kW CW transmission through >1km of fibre and of MW peak power
• sensing: high performing fibres for interferometric applications and for mid-IR spectroscopy
• metrology: environmentally insensitive fibres
• nonlinear endoscopy
• short distance data communication (e.g. 5G networks or intra-data centre interconnection)